Technical Field
The present invention relates to a vacuum pressure control apparatus including an annular elastic seal member formed with a valve seat, and a doubly eccentric butterfly valve element which comes into contact with or separates from the valve seat, the butterfly valve element being placed on a pipe connecting a vacuum chamber and a vacuum pump and being rotatable in a first direction to change vacuum pressure in the vacuum chamber.
Related Art
A vacuum pressure control apparatus used conventionally is disclosed, for example, in Patent Document 1, an application of which was filed by the present applicant. A poppet valve of this vacuum pressure control apparatus is adequate to control vacuum pressure. However, this poppet valve has a small conductance and thus could not be made compact.
Therefore, it is conceivable to use a doubly eccentric butterfly valve element in order to improve the conductance and to control the pressure of the vacuum chamber under near atmospheric pressure. For control of pressure under vacuum, it is necessary to delicately adjust an opening (an opening degree) of the valve element near a valve fully closed position, that is, just before the valve is completely closed. This adjustment is more difficult as differential pressure between an upstream side and a downstream side of the valve is larger.
The reason to use the doubly eccentric butterfly valve element is that, just before the valve is closed, the opening of the valve can be changed slowly with respect to a rotation angle of a valve element, thereby facilitating pressure control by the valve near a valve closed position.